Thermoablation consists of destroying target tissues by increasing the temperature of cells beyond an irreversible damage threshold. This threshold is linked to exposure time at a given temperature; in the case of temperatures comprised between 50° C. and 60° C. the time is a few minutes whereas from 60° C. above cell death is almost instantaneous. The temperature increase is obtained by dispensing energy into the target tissues by more or less invasive applicators. The forms of energy that are commonly used for thermoablation comprise mechanical waves, radio-frequency currents, infrared radiation, and microwaves.
One of the most promising forms of energy for thermoablation is currently microwave energy, which provides an excellent compromise between the efficiency of the energy transfer and the depth of penetration into the biological tissues. The dispatch of microwave energy into tissues intended for thermoablation occurs by percutaneous, endoscopic, laparotomic or laparoscopic insertion of interstitial applicators consisting of a coaxial antenna comprising an internal conductor, a layer of dielectric material that covers the entire length of the internal conductor, an external conductor that coaxially covers the layer of dielectric material and the internal conductor, except for a distal end portion thereof, constituting the radiant end of the antenna. The design of antennas for thermoablation has to consider certain constructional requirements linked to the use that is made thereof, in particular: biocompatibility, great mechanical resistance, spherical coagulative necrosis, a diameter of the antenna that is as small as possible.
The antenna for guaranteeing a spherical coagulative necrosis requires both a radiation figure that is in turn spherical and a cooling system for dissipating the generated heat of the supply line of the antenna. At the operating frequencies of the microwave thermoablation system the transit power through the coaxial cable is characterised by great attenuation to which heating of the coaxial cable corresponds. The heat generated could cause necrosis of the tissues in contact with the external stem of the antenna over the entire length thereof. The presence of a cooling circuit of the supply line enables the heat to be removed and the eccentricity of the necrosis to be thus reduced.